Following the sighting by Tombc52, the analysis by Apskip and DrRox, and a trip I just took to both Liechtenstein and Zermatt, here's the itinerary I'm confident LaKisha/Jennifer were on the afternoon of 7 Dec:
IR 1776 from Sargans to Zurich, 1239-1348
IC 826 from Zurich to Visp, 1402-1602
R 249 from Visp to Zermatt, 1625-1734
Sargans is one of the two Swiss Rail stations that serve Liechtenstein, but the other one, Buchs, would not involve the sisters travelling on the Chur-Zurich train on which they were sighted.
If there was a normal Pit Stop (as opposed to a rolling Pit Stop or a TBC) in Vienna, teams would necessarily have flown to get to Liechtenstein. Herbert/Nathaniel were seen racing in Vienna at 11.30am on 6 Dec. If there is a normal Pit Stop that lasts 12h or more, they will not be able to make the last train to Feldkirch (Austrian jump-off point to Liechtenstein) at 11.56pm. The first train option for the next day is a direct, from 7.14am to 1.49pm. However, this is incongruent with the sisters leaving Liechtenstein more than an hour before, before 12.39pm. Teams in this scenario therefore must have flown between Vienna and Liechtenstein.
The first possible itinerary is:
AA 8094 from Vienna to Zurich Airport, 0736-0846
IC 716 from Zurich Airport to Zurich, 0913-0923
IC 565 from Zurich to Sargans, 0937-1032